** commit or rollback - diff

If I have not done a dml transaction in a session :
no update,
delete or insert etc. I have only done select and some of the objects
can be over a db link. So I can do a commit or rollback so that no
transaction is pending in my session. My question is : is there any
difference in such case between the behaviour of commit and rollback.
When there is no data as such to commit or rollback. I
am thinking it is better to do rollback since it has to do less. Am I
wrong. Any observation. Thanks for help. Thanks